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Data Sheet

AD9833

Rev. F | Page 17 of 21

APPLICATIONS INFORMATION

Because of the various output options available from the part,

the AD9833 can be configured to suit a wide variety of applications.

One of the areas where the AD9833 is suitable is in modulation

applications. The part can be used to perform simple modulation,

such as FSK. More complex modulation schemes, such as

GMSK and QPSK, can also be implemented using the AD9833.

In an FSK application, the two frequency registers of the AD9833

are loaded with different values. One frequency represents the

space frequency, while the other represents the mark frequency.

Using the FSELECT bit in the control register of the AD9833, the

user can modulate the carrier frequency between the two values.

The AD9833 has two phase registers, which enables the part to

perform PSK. With phase-shift keying, the carrier frequency is

phase shifted, the phase being altered by an amount that is

related to the bit stream being input to the modulator.

The AD9833 is also suitable for signal generator applications.

Because the MSB of the DAC data is available at the VOUT pin,

the device can be used to generate a square wave.

With its low current consumption, the part is suitable for

applications in which it can be used as a local oscillator.

GROUNDING AND LAYOUT

The printed circuit board (PCB) that houses the AD9833 should be

designed so that the analog and digital sections are separated

and confined to certain areas of the board. This facilitates the

use of ground planes that can be separated easily. A minimum

etch technique is generally best for ground planes because it gives

the best shielding. Digital and analog ground planes should be

joined in one place only. If the AD9833 is the only device requiring

an AGND-to-DGND connection, then the ground planes should

be connected at the AGND and DGND pins of the AD9833. If

the AD9833 is in a system where multiple devices require AGND-

to-DGND connections, the connection should be made at one

point only, a star ground point that should be established as

close as possible to the AD9833.

Avoid running digital lines under the device as these couple noise

onto the die. The analog ground plane should be allowed to run

under the AD9833 to avoid noise coupling. The power supply

lines to the AD9833 should use as large a track as possible to

provide low impedance paths and reduce the effects of glitches

on the power supply line. Fast switching signals, such as clocks,

should be shielded with digital ground to avoid radiating noise

to other sections of the board.

Avoid crossover of digital and analog signals. Traces on opposite

sides of the board should run at right angles to each other. This

reduces the effects of feedthrough through the board. A microstrip

technique is by far the best, but it is not always possible with a

double-sided board. In this technique, the component side of

the board is dedicated to ground planes, and signals are placed

on the other side.

Good decoupling is important. The AD9833 should have supply

bypassing of 0.1 μF ceramic capacitors in parallel with 10 μF

tantalum capacitors. To achieve the best performance from the

decoupling capacitors, they should be placed as close as possible

to the device, ideally right up against the device.

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